It is already known, in electromotors for centrifugal, underwater or liquid pumps, and also in large electrical machinery, generators and turbo-generators, to provide cooling systems by means of which the stator and/or the rotor or, alternatively, the whole of the electromotor, can be cooled using liquid or gaseous coolants, such as compressed air, water or oil, and in which liquid coolants can be vaporised, sprayed or atomised, and in which provision can be made for special devices such as cooling grooves, cooling slots, cooling rings, tubular conductors or baffle equipment, in order to achieve an especially good cooling effect on certain components. It is also possible to equip the cooling system with control and distributing devices and with monitoring installations.
It is also known to provide circulation of the cooling fluid or the cooling gas in the cooling circuit by means of a pump which is driven directly from the shaft of the motor. A disadvantage of these known cooling systems is that, because the coolant pump is driven directly from the shaft of the motor, it is not possible to achieve a constant circulation of the cooling fluid with a consequent constant cooling effect, owing to the fact that the degree of cooling rises or falls with the speed of revolution of the motor to be cooled.
In the case of induction motors designed as variable speed motors, it must be possible to change the speed of rotation frequently within wide limits, as a result of which the heat losses in the motor usually undergo a considerable increase as the speed of rotation falls. With variable speed induction motors, the problem therefore arises that, when the coolant pump is driven from the shaft of the motor, the cooling effect drops sharply precisely when a particularly large amount of heat losses have to be dissipated if overheating of the motor is to be avoided.